INTRODUCTION
Chelonians (along with the tuatara) are the most ancient of all living reptiles and evolved over 200 million years ago (Evans 1986). They predate dinosaurs and their primeval shelled appearance has made them a popular and fascinating pet with none of the sinister connotations of snakes.
Their survival through evolution is related to their ability to withdraw their head inside the protective armor of their shell. Consequently, they have not needed to diversify, which is why flying and fossorial turtles do not exist (King 1996).North America has a quarter of the world's chelonians (Evans 1986) while Europe has only two species of freshwater turtle and three terrestrial species. Australia has no terrestrial species and only has aquatic sidenecks.
Longevity
Of all the reptiles, chelonians live the longest, with many living well over 50 years in captivity. Larger species like the Galapagos tortoise (Geochelone nigra) have been recorded to live for over 150 years (Bellairs 1969d; Pough et al. 1998a, 2002).
Size
The largest chelonia are the marine Leathery turtle (Dermochelys coriacea), which can weigh 680 kg and the Galapagos Giant tortoise weighing 263 kg. Larger size does confer the advantage of thermal inertia and the ability to store fat for lean periods. The majority of species reach a maximum carapace length of 30 cm although smaller species like the Madagascar Spider tortoise (Pyxis arach- noides) has an adult shell length of only 10 to 15 cm (Bellairs 1969d).